This invention relates to systems for printing of the type referred to as screen printing. In such systems, when printing cylindrical objects a screen is moved in linear motion synchronized with the surface of the rotating cylindrical object to be printed, and a squeegee establishes the contact between the screen and the surface. The object is sometimes rotated by friction or may be driven by a pulley and cable or a pinion and rack synchronized to the screen printing plate movement. Ink or the like is forced by the squeegee through openings in the screen onto the surface.
It is often desirable to print in several colors and this is accomplished by allowing the first color applied to dry and then applying one or more additional colors in successive printing and drying steps. To accomplish this in a satisfactory fashion, particularly where cylindrical objects are being printed, registration of the successive colors must be accomplished so that the overall multi-color effect will be achieved, and so that, in runs of several objects to be printed, each printed surface will appear the same on each object.
Various inks, epoxies, waxes, precious metals, ceramic colors, thermoplastic materials, etc., may be applied. Drying may be accomplished using air movement, heat, U.V. curing, thermoplastic inks, kilns, lehrs, etc.
The foregoing objects can be achieved using automatic machines wherein an object is held in a fixture and the fixture is moved through successive printing and drying stations without disturbing the position of the object in a fixture. Such automatic machines are expensive, however, and semi-automatic machines are more economical in many situations.
With the semi-automatic equipment, an object is placed in a fixture, a color is printed, the object is then removed for drying, and the object is then remounted in a fixture for printing of the next color. Certain objects, such as rectangular objects, do not present a problem as far as registration is concerned since they define distinct side edges which will serve as reference points to insure that the object is always placed in the same position in a fixture. Similarly, a cylindrical plastic object can be molded with an indent or the like which will provide a mating relationship with a fixture so that the object will always be mounted in the same position. Other distinctive parts of an object such as a handle, threaded area, or slot formed in an object can also be used as means for mechanically coupling the object in the fixture so that, with the fixture drive synchronized with the screen carriage movement, registration will be achieved.
Some objects, however, are truly symmetrical and do not have "built-in" reference points. These include glass and plastic drinking tumblers, wine glasses, open end dispensing tubes, wax candles and some medical items. For cosmetic and other reasons, it is also not desirable to form indentations and the like in the objects solely to provide a reference point for registration.
U.S. Pat. No. 5,553,547 includes a discussion of systems using a pointed indicator, such as a stylus, which is manipulated by an operator after a first color has been printed. The second and successive colors are printed after the indicator has been located at a predetermined reference point established by the first printing. The patent also discusses use of an infrared light source and detector to locate the reference point. Finally, the patent discloses a system wherein a laser beam generator and detector is employed which is stated to be less time consuming and more accurate than the other systems discussed.